Circularly polarized beam splitter based on helical double-core antiresonant hollow-core fiber

Abstract

The present invention discloses a circularly polarized light beam splitter based on a helical dual-core anti-resonant hollow-core fiber. The cladding of the fiber includes six circular nested tubes and two elliptical nested tubes that separate the interior of the fiber into two symmetrical cores. When light propagates in one core, it is mainly coupled to the other core through the gap between the elliptical tubes. At the same time, the fiber is designed into a dual-core helical chiral structure along the light transmission direction, so that the four supermode fundamental modes of the fiber are converted into circularly polarized modes, namely, left-circularly polarized odd mode and coupled mode, and right-circularly polarized odd mode and even mode. Subsequently, the effective refractive index of the two odd and even modes with different polarizations is adjusted by optimizing the fiber structure parameters to obtain a suitable mode coupling length, thereby achieving a good beam splitting effect for circularly polarized light. Finally, by adjusting the size of the nested inner tubes, the coupling between the core high-order mode and the cladding mode is increased, so that the high-order mode confinement loss is increased, thereby ensuring the single-mode performance of the fiber.

Description

Technical Field

[0001] The invention belongs to the technical field of polarization beam splitters, and in particular relates to a double-core hollow-core anti-resonance optical fiber polarization beam splitter with a spiral structure. Background Art

[0002] Solid-core optical fibers based on quartz glass are limited by inherent Rayleigh scattering, dispersion, material absorption, and nonlinear effects, making it impossible to achieve superior optical transmission performance. The emergence of hollow-core optical fibers has overcome these limitations. Hollow-core optical fibers include photonic crystal hollow-core optical fibers and antiresonant hollow-core optical fibers. Photonic crystal hollow-core optical fibers confine light in the hollow core through the photonic bandgap principle, but their development is also limited by their inability to reduce surface scattering losses. Compared to photonic crystal hollow-core optical fibers, antiresonant hollow-core optical fibers can overcome the shortcomings of photonic crystal hollow-core optical fibers' surface scattering losses while ensuring light transmission in the air medium, and have a simpler cladding structure. Therefore, antiresonant hollow-core optical fibers have great potential. Currently, the lowest loss of antiresonant hollow-core optical fibers has been as low as 0.8dB / km, making them a strong competitor to solid-core optical fibers.

[0003] Optical passive components are an indispensable component of fiber optic systems. Directly connecting solid-core fiber components to antiresonant hollow-core fibers can severely impact the stability and efficiency of the optical system due to significant Fresnel reflections or mode mismatch at the silica-air interface at the junction. Therefore, research on optical passive components based on hollow-core antiresonant fibers is of great significance.

[0004] Polarization beam splitters play an important role in optical passive devices. They can spatially split a single beam of light into two orthogonal polarizations and are widely used in information processing, communications, sensing, and other fields. Currently, linear polarization beam splitters based on dual-core antiresonant hollow-core fibers (ARCFs) have been extensively studied. The beam splitting principle is as follows: ARCFs generate four linearly polarized supermode fundamental modes, generally divided into odd and even modes polarized in the x-direction, and odd and even modes polarized in the y-direction. Odd and even modes with the same polarization direction couple with each other as they propagate through the fiber. Due to the different supermode coupling lengths for different polarization directions, after light propagates a certain distance in one core, one polarization is fully coupled into the other core, while the other polarization remains in the original core, achieving linear polarization beam splitting. Currently, the application of ARCF-based linear polarization beam splitters has several challenges, such as potential polarization axis misalignment during connection and squeezing during practical application, resulting in polarization errors and noise. Using circular polarization beam splitters can mitigate these issues. Summary of the Invention

[0005] The object of the present invention is to provide a circularly polarized light beam splitter based on a spiral double-core anti-resonant hollow-core optical fiber with low loss, good single-mode performance and good beam splitting effect.

[0006] To solve the above technical problems, the present invention is achieved through the following technical solutions:

[0007] The circularly polarized beam splitter based on helical double-core antiresonant hollow-core fiber has the following characteristics:

[0008] The optical fiber comprises a circular outer tube and eight nested tubes that inscribe the circular outer tube. The eight nested tubes are spaced apart on the inner wall of the circular outer tube and fused to the circular outer tube at the inscription points. The eight nested tubes are arranged in a spiral pattern along the axial direction of the optical fiber. The eight nested tubes are arranged symmetrically along the cross section of the optical fiber, with one pair of nested tubes being elliptical and the remaining being circular. The two elliptical nested tubes divide the optical fiber into two symmetrical cores, designated as core A and core B. When light is injected into core A, due to the different coupling lengths of different circularly polarized light, after propagation for a certain distance, left-circularly polarized light and right-circularly polarized light of a specific wavelength are completely separated in cores A and B, respectively, thus functioning as a circularly polarized beam splitter.

[0009] Furthermore, the eight nested tubes form a left-handed helical structure along the axial direction of the optical fiber, that is, along the light propagation direction, with a twist rate α of 0.9 rad / mm.

[0010] Furthermore, the included angle between the central axes of the cross sections of two adjacent nested tubes is 45°.

[0011] Furthermore, the nested tube material is quartz glass, and the refractive index is determined by the Sellmeier formula; the other areas in the optical fiber are air, and the refractive index is 1.

[0012] In order to get rid of the defects of the linear polarization beam splitter, the present invention adopts a dual-core anti-resonant hollow-core optical fiber with a helical structure. Unlike the traditional non-helical linear polarization beam splitter, the helical structure makes the supermode in the dual-core anti-resonant hollow-core optical fiber a circular polarization mode. The four supermodes generated are left circular polarization odd mode, left circular polarization even mode, and right circular polarization odd mode and right circular polarization even mode. When the phase difference between the modes in the two cores is π, it is an odd mode, and when the phases are the same, it is an even mode. When light propagates in the fiber core, modes with the same polarization direction will couple with each other. According to the symmetric waveguide mode coupling theory, the coupling length of modes with different polarization directions can be calculated by the following formula:

[0013]

[0014] in, represents the coupling length corresponding to the circularly deflected mode, Δβ LCP / RCPThe effective refractive index difference between the odd and even modes of the corresponding circularly polarized mode is 0.5 or 2, which results in the best beam splitting effect.

[0015] To achieve the aforementioned effects, the helical dual-core antiresonant hollow-core fiber designed in this invention has a radius (i.e., the inner radius R of the circular outer sleeve) of 36 μm. It is composed of eight evenly arranged nested tubes forming a cladding structure, and the wall thickness t of each nested tube is 0.55 μm. Each nested tube comprises an outer tube and an inner tube, and the inner tube is inscribed within the inner wall of the outer tube, with the tangent point being the same as the tangent point between the outer tube and the circular outer sleeve. The outer and inner cross-sections of the elliptical nested tubes are both elliptical, while the outer and inner cross-sections of the circular nested tubes are both circular.

[0016] The outer tube of the nested elliptical tube has a major semi-axis length of 17.875 μm and a minor semi-axis length of 8.3 μm; the inner tube of the nested elliptical tube has a major semi-axis length of 10 μm and a minor semi-axis length of 5 μm. This divides the interior of the optical fiber into two symmetrical cores, spatially splitting left-circularly polarized light and right-circularly polarized light.

[0017] The outer tube diameter (D1) of the circular nested tubes is 18 μm, the inner tube diameter (D2) of the two circular nested tubes not adjacent to the two elliptical nested tubes is 10 μm, and the inner tube diameter (D3) of the four circular nested tubes adjacent to any one of the elliptical nested tubes is 6 μm. Specifically, the central axis direction of the cross-section of the two elliptical nested tubes can be defined as the Y-axis direction, that is, the two elliptical nested tubes are distributed in the Y-axis direction; then the circular nested tubes not adjacent to the two elliptical nested tubes are circular nested tubes distributed in the X-axis direction perpendicular to the Y-axis, and the circular nested tubes adjacent to any one of the elliptical nested tubes are circular nested tubes distributed in directions that form a 45° angle with both the Y-axis and the X-axis. The different inner tube diameters allow the high-order modes of the fiber core to couple more with the cladding modes, increasing the loss of the high-order modes. At the same time, the nested structure reduces the loss of the fiber core fundamental mode, thereby improving the high-order mode suppression ratio, where the high-order mode suppression ratio is the ratio of the lowest loss of the high-order mode to the highest loss of the fundamental mode.

[0018] The beneficial effects of the present invention are embodied in:

[0019] The present invention discloses a circularly polarized light beam splitter based on a dual-core antiresonant hollow-core fiber. Its cladding includes six circular nested tubes and two elliptical nested tubes, each nested at an angle of 45°, forming a circular distribution. The two elliptical tubes separate the interior of the fiber into two symmetrical cores. When light propagates in one core, it is primarily coupled to the other core through the gap between the elliptical tubes. Simultaneously, the fiber is designed into a dual-core helical chiral structure along the light transmission direction, converting the fiber's four supermode fundamental modes into circularly polarized modes: left-handed circularly polarized odd and even modes, and right-handed circularly polarized odd and even modes. Subsequently, the effective refractive indexes of the two odd and even modes of different polarizations are adjusted by optimizing the fiber structural parameters to obtain an appropriate mode coupling length, thereby achieving excellent beam splitting effects for circularly polarized light. Finally, by adjusting the size of the nested inner tubes, the coupling between the core high-order modes and the cladding modes is increased, thereby increasing the high-order mode loss and thus ensuring the single-mode performance of the fiber. Ultimately, a helical hollow-core fiber circularly polarized light beam splitter capable of spatially splitting circularly polarized modes and exhibiting excellent single-mode characteristics is achieved. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the following briefly introduces the drawings required for describing the embodiments.

[0021] Figure 1 A schematic cross-sectional view of a circularly polarized beam splitter according to an embodiment of the present invention;

[0022] Figure 2 A schematic diagram of a three-dimensional spiral structure of a circularly polarized beam splitter provided in an embodiment of the present invention;

[0023] Figure 3 A graph showing the relationship between the polarization mode coupling length ratio and wavelength of the circularly polarized beam splitter provided in an embodiment of the present invention;

[0024] Figure 4 A graph showing the relationship between the normalized power of the circularly polarized beam splitter provided in an embodiment of the present invention and the transmission length;

[0025] Figure 5 A graph showing the relationship between the polarization extinction ratio of the circularly polarized beam splitter provided in an embodiment of the present invention and wavelength;

[0026] Figure 6 This is a graph showing how the high-order mode suppression ratio of the circularly polarized beam splitter provided in an embodiment of the present invention varies with wavelength. DETAILED DESCRIPTION

[0027] The technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, rather than all the embodiments.

[0028] like Figure 1 Figure 1 shows a cross-sectional schematic diagram of the circularly polarized beam splitter designed based on a helical dual-core antiresonant hollow-core fiber proposed in this invention. The fiber comprises a circular outer sleeve and eight nested tubes inscribed within the outer sleeve. The eight nested tubes are spaced apart on the inner wall of the outer sleeve and fused to the outer sleeve at the inscribed points. The eight nested tubes are arranged symmetrically along the cross-section of the fiber, with one pair of nested tubes being elliptical and the remaining tubes being circular.

[0029] like Figure 2 As shown, eight nested tubes form a left-handed helical structure along the fiber's axial direction (i.e., along the direction of light propagation), with a twist rate of 0.9 rad / mm. This helical structure generates left-hand and right-hand circular polarization modes within the fiber. Two elliptical nested tubes divide the fiber into two symmetrical cores, designated core A and core B, spatially splitting the two circular polarization modes. The wall thickness t of each nested tube is 0.55 μm. The semimajor axis (R1) and semiminor axis (R2) of the outer elliptical nested tube are 17.875 μm and 8.3 μm, respectively. The semimajor axis (R3) and semiminor axis (R4) of the inner elliptical nested tube are 10 μm and 5 μm, respectively. With these parameters, the elliptical nested tube minimizes the supermode fundamental mode loss generated by the fiber while maintaining an appropriate coupling length ratio. The outer diameter D1 of the circular nested tubes is the same at 18 μm. The circular nested tubes in the x-direction (i.e., the two circular nested tubes not adjacent to either elliptical nested tube) have a larger inner diameter of 10 μm. The circular nested tubes in the 45° direction (i.e., the circular nested tubes adjacent to any elliptical nested tube) have a smaller inner diameter of 6 μm. With this structural design, higher-order modes generated within the fiber are more likely to couple with the cladding tube modes in the 45° direction. This improves higher-order mode loss while maintaining low fundamental mode loss, thereby enhancing the higher-order mode suppression ratio.

[0030] like Figure 3 As shown in Figure 1, the coupling length and coupling length ratio of two circularly polarized modes in the helical double-core hollow-core antiresonant fiber are calculated based on coupled mode theory and transformation optics. The mode coupling length can be calculated by formula (1), and the mode coupling length ratio can be calculated by the following formula:

[0031]

[0032] The length of the designed circular polarization beam splitter When the CLR value is 0.5 or 2, the length of the circular polarization beam splitter is the shortest and can ensure a good beam splitting effect. In this embodiment, when the operating wavelength increases from 1.35μm to 1.65μm, From 17.6cm to 9.1cm, From 8.5cm to 5.9cm, CLR increases from 0.4829 to 0.6483. Set the length of the circular polarization beam splitter to 15.6cm. At this time, its operating wavelength is at 1.4μm. is 15.6cm, It is 8.1cm and the CLR is 0.5192.

[0033] According to mode coupling theory, the relationship between the polarization mode output power and propagation distance in an optical fiber can be calculated by the following formula:

[0034]

[0035] in, Indicates the output power of left-handed polarization mode or right-handed polarization mode, P in Represents input power, and L represents propagation distance. Based on the mode coupling length of the designed circularly polarized beam splitter at 1.4μm, the output power of different polarization modes is calculated, and the results are as follows: Figure 4 As shown in Figure 2, the best beam splitting effect is achieved at 15.6 cm.

[0036] The polarization extinction ratio is an important indicator for evaluating the beam splitting performance of a polarization beam splitter and can be calculated using the following formula:

[0037]

[0038] The polarization extinction ratio of the circularly polarized beam splitter in the embodiment is calculated by formula (4) to obtain the relationship between the polarization extinction ratio and the wavelength. The result is as follows: Figure 5 When |ER|>20dB, the polarization beam splitter is considered to have effective beam splitting performance. In the embodiment, when the operating wavelength is between 1.374 and 1.428 μm, the polarization extinction ratio is greater than 20 dB, that is, the designed circularly polarized beam splitter has an operating bandwidth of 54 nm.

[0039] The designed circularly polarized beam splitter should have good single-mode performance. Its single-mode performance can be calculated based on the high-order mode suppression ratio of the optical fiber. The high-order mode suppression ratio is obtained by dividing the minimum loss of the high-order mode by the maximum loss of the fundamental mode. In this embodiment, the relationship between the high-order mode suppression ratio (homer), the minimum loss of the high-order mode (CLhm), and the maximum loss of the fundamental mode (CLfm) and the wavelength is as follows: Figure 6 Structurally, by reducing the size of the inner tubes of the nested tubes at 45°, the coupling between the high-order modes and these inner tube modes is increased, thereby increasing the high-order mode loss and thus improving the high-order mode suppression ratio. Ultimately, a high-order mode suppression ratio greater than 100 is achieved in the range of 1.30 to 1.55 μm.

[0040] This specification selects and specifically describes these embodiments in order to better explain the principles and practical applications of the present invention, so that those skilled in the art can better understand and utilize the present invention.

Claims

1. A circularly polarized beam splitter based on a helical double-core anti-resonant hollow-core fiber, characterized by: The optical fiber includes a circular outer tube and eight nested tubes inscribed in the circular outer tube; the eight nested tubes are arranged at intervals on the inner wall of the circular outer tube; The eight nested tubes form a left-handed helical structure along the axial direction of the optical fiber, that is, along the direction of light propagation, with a twist rate α of 0.9 rad / mm; The eight nested tubes are arranged symmetrically along the cross section of the optical fiber, with one pair of nested tubes being elliptical and the remaining being circular. The outer semi-major axis R1 of the elliptical nested tubes is 17.875 μm, the semi-minor axis R2 is 8.3 μm, and the inner semi-major axis R3 is 10 μm, and the semi-minor axis R4 is 5 μm. The outer diameter D1 of the circular nested tubes is 18 μm, the inner diameter D2 of the two circular nested tubes that are not adjacent to either elliptical nested tube is 10 μm, and the inner diameter D3 of the four circular nested tubes adjacent to any elliptical nested tube is 6 μm. The included angle between the central axes of the cross sections of two adjacent nested tubes is 45°. Two elliptical nested tubes divide the optical fiber into two symmetrical cores, which are marked as A core and B core respectively; When in use, light is injected from core A. Due to the different coupling lengths of different circularly polarized lights, after a certain distance of transmission, the left circularly polarized light and the right circularly polarized light of a specific wavelength are completely separated in core A and core B, realizing the function of a circularly polarized beam splitter.

2. The circularly polarized beam splitter based on helical double-core antiresonant hollow-core fiber according to claim 1, characterized in that: The inner radius R of the circular outer sleeve is 36 μm.

3. The circularly polarized beam splitter based on helical dual-core antiresonant hollow-core fiber according to claim 1, characterized in that: The nested tube is made of quartz glass, and its refractive index is determined by the Sellmeier formula; the other areas in the optical fiber are air, and its refractive index is 1.

4. The circularly polarized beam splitter based on helical dual-core antiresonant hollow-core fiber according to claim 1, characterized in that: The wall thickness t of the nested tubes is 0.55 μm.

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